Biggest ones

. . From what I can see, I would say ... isaan ? . . . 

Not that I am particularly interested because I know that generalization is not truth ... but there is a thread now running on a thai webboard about where in Thailand you can find the boys with the biggest ones ....

The general opinion is that the Northern boys have it generally short and round (about 5"/ 6"), and if you find bigger ones, they probably come from the mountains or from beyond Mae Sai (meaning : Burma).
You will get the right size (6 / 7 ") from the boys in the center of the country.
Num isaan have it long (from 7" upwards) and big, swarthy (almost black), stiff, and they know how to use it (เอาเก่ง).
The biggest ones come from the south : 7" to 8", fleshy, dark-coloured, but rather violent and not inclined to wheedling ...

Our thai counterparts seem to have the same feeling as us, but they do prefer northern boys because they are neat and clean, handsome and unblemished. And of course some dek nua have it quite big too ...
Which I can personnally confirm ...